Portable communication devices, such as cellular telephones, typically use one or more power amplifiers to amplify an information signal prior to transmission. Modern communications systems often use both phase and amplitude modulation to boost information transmission rates, generally at the expense of power consumption. Often, a linear power amplifier is used for systems that use phase and amplitude modulation (such as systems that employ code division multiple access (CDMA) or enhanced data rates for GSM evolution (EDGE)), while a non-linear power amplifier is used for systems that employ phase only modulation (e.g., a constant envelope modulation system such as Gaussian mean shift keying (GMSK) modulation).
Typically, a device is designed to use either a linear power amplifier or a non-linear power amplifier based on the type of communication network for which the device is being designed. The power amplifier is usually implemented as one or more stages of transistors and related circuitry. In most applications, the operating point of the power amplifier is set by providing a bias current or voltage to at least one of the terminals of at least one of the stages of the power amplifier. In the case of a bipolar junction transistor (BJT) the bias current is normally applied to the base terminal of the transistor to control how the transistor will conduct between its collector and emitter terminals. In a typical implementation, the power amplifier comprises one or two driver stages followed by an output stage.